Proximity switches are generally known in the art and have been widely applied to sense the position of a moving machine element. Such known proximity sensors frequently employ a probe which is rigidly mounted to a fixed portion of an associated machine element and extending toward the intended target such as splines on a rotating shaft. The probe terminates adjacent the shaft and contains a sensor which is electrically interconnected with a control circuit and is inductively or magnetically coupled to the target as it rotates in close proximity to the sensor.
Various technologies have been employed to effect a coupling between the sensor and moving target. For example, some are known to utilize an oscillator drive circuit in combination with an induction tank circuit. The tank circuit includes an induction coil within the probe as a means for sensing the presence of a metal object. The induction coil is constructed such that it generates a magnetic field in the area surrounding the coil. The magnetic field induces eddy currents in a conductive object within the field. Such objects are known in the art as targets. Once a target comes within the magnetic field of the coil, energy is drawn from the induction coil. A typical induction proximity switch selects components of the oscillator and tank circuit to ensure that oscillations occur when a target is absent from the magnetic field of the induction coil. When a target comes within the magnetic field, the oscillation amplitude is attenuated due to the loss of energy caused by eddy currents induced in the target. The amount of oscillation attenuation is directly related to the distance between the target and the induction coil.
Another common approach is the use of a Hall sensor within the probe and a concentric permanent magnet ring carried on the rotating shaft. Hall effect sensors have, however, proven to be application limited in as they tend to be fragile and range limited and are thus unsuitable in some applications. Furthermore, all sensors do not provide field shaping or transformer action needed to concentrate magnetic flux field for extremely sensitive contactless measurement.
Although more robust devices are known, they tend to be less sensitive, requiring extremely close positioning with respect to the rotating target. Furthermore, they may require a uniquely shaped target and be unsuitable for detecting the presence of standard design machine elements such as gear teeth and shaft splines. Furthermore, the requirement of close positioning raises the potential for misassembly wherein the sensor will be positioned too far from the target to effectively sense its rotation or, conversely, be positioned too closely whereby it is contacted and damaged by the rotating target.
Another disadvantage of many prior art sensors is that they cannot delineate the direction of passage of a target through the sensor's sensing region and thereby cannot discern direction of shaft rotation.
A final problem with typical prior art sensors is that they are pressure limited (typically 50 psi) and cannot tolerate operation within a high pressure environment such as hydraulic equipment.